Recently, the usage environment of a roll has become much harsher as hot rolling techniques for manufacturing steel sheets progress. Also, recently, the production amount of steel sheets such as high strength steel sheets and thin steel sheets which require a high rolling load is increasing. Therefore, there is a growing tendency for surface deterioration and chipping scars to occur due to fatigue of a rolling surface of a roll for hot rolling. “Roll for hot rolling” refers to a work roll to be used for finish rolling.
Nowadays, a high speed steel roll whose abrasion resistance is increased by adding V to steel in an amount of several percent to form hard carbides in a large amount is often used in hot rolling.
For example, Japanese Unexamined Patent Application Publication Nos. 4-365836 and 5-1350 propose roll outer layer materials of high speed steel for hot rolling. The roll outer layer material for hot rolling described in JP '836 contains C: 1.5% to 3.5%, Ni: 5.5% or less, Cr: 5.5% to 12.0%, Mo: 2.0% to 8.0%, V: 3.0% to 10.0%, and Nb: 0.5% to 7.0%. In addition, the roll outer layer material for hot rolling described in JP '836 contains Nb and V so that the contents of Nb, V, and C satisfy a specified relationship and so that the ratio between the contents of Nb and V is within a specified range. It is said that, by using this material, since the segregation of hard carbides in the outer layer material is inhibited even if a centrifugal casting method is used, it is possible to obtain a roll outer layer material for hot rolling excellent in terms of abrasion resistance and crack resistance.
The roll outer layer material for hot rolling described in JP '350 contains C: 1.5% to 3.5%, Cr: 5.5% to 12.0%, Mo: 2.0% to 8.0%, V: 3.0% to 10.0%, and Nb: 0.5% to 7.0%. In addition, the roll outer layer material for hot rolling described in JP '350 contains C, Nb, and V so that the contents of Nb, V, and C satisfy a specified relationship and so that the ratio between the contents of Nb and V is within a specified range. By using this material, since the segregation of hard carbides in the outer layer material is suppressed even if a centrifugal casting method is used, there is an increase in abrasion resistance and crack resistance of the roll for hot rolling. Therefore, it is said the technique described in JP '350 contributes to an increase in efficiency of hot rolling.
However, the usage environment of a roll for hot rolling has become harsher with improved quality and increased productivity of products manufactured by performing hot rolling. Moreover, since the requirement for the surface quality of products manufactured by performing hot rolling has become more severe and, since the number of steel sheets manufactured using continuous rolling processes is increasing, inhibiting the fatigue damage of a roll surface such as surface deterioration is a larger problem to be solved than suppressing abrasion of a roll.
To solve such a problem, Japanese Unexamined Patent Application Publication No. 2009-221573 proposes a composite roll produced through centrifugal casting method, which contains C: 2.2% to 2.6%, Cr: 5.0% to 8.0%, Mo: 4.4% to 6.0%, V: 5.3% to 7.0%, and Nb: 0.6% to 1.3%, and in which the contents of C, Mo, V, and Nb are controlled so that the contents of Mo+V and C−0.24V−0.13Nb are within specified ranges. It is said that this composite roll produced through centrifugal casting method has a roll surface layer excellent in terms of fatigue resistance in a hot rolling environment.
However, recently, rolling technology has been progressing at a high speed to manufacture rolled steel sheets of higher quality and higher grade. Also, it is strongly required to save rolling costs. As described above, the usage environment of a roll for hot rolling has become harsher. In particular, the occurrence of fatigue damages on the surface of a roll for hot rolling such as surface deterioration and chipping which are caused mainly by the fracturing of a large-size carbide on the surface of a roll for hot rolling is seen as a problem. It is confirmed that the fatigue damages described above still occur even when using the technique described in JP '573.
It could therefore be helpful to provide a roll outer layer material for hot rolling having a surface layer excellent in terms of fatigue resistance (“fatigue resistance” refers to the quality of inhibiting fatigue damages such as surface deterioration and chipping which occur on the surface layer of a roll when hot rolling is performed), and to provide a composite roll for hot rolling produced through centrifugal casting (hereinafter, also called composite roll for hot rolling) having an outer layer composed of the roll outer layer material.